The present disclosure relates to power switching converters providing a constant current to a load, for example a battery, with high efficiency. The present invention more particularly relates to a switched voltage and current regulator whose load should be isolated from the supply source of the regulator, frequently the 220-volt mains.
For example, a conventional power switching converter is the flyback converter wherein a transformer with a primary winding and a secondary winding is provided for isolating the load from the voltage source. The primary winding is connected to the voltage source through a power switch while the secondary winding is connected to a load by means of a diode and a filtering capacitor is connected in parallel to the load.
When the switch switches-on, a first current flows though the primary winding and increases from an initial value as a function of the values of the voltage source and of the inductance provided by the primary winding. During this time, no current flows through the secondary winding because the diode is reverse biased and the power is stored in the core of the transformer.
When the switch switches-off, the current flowing through the primary winding is abruptly switched-off and the power that was just stored in the core is transferred into the secondary winding. A second current on the secondary winding abruptly reaches a peak value equal to the peak current reached by the first current multiplied by the ratio between the number of turns of the primary winding and the secondary winding, when the switch is switched-off. The second current starts to decrease as a function of the inductance of the secondary winding and of the voltage across the load.
The amount of power transferred from the primary winding to the secondary winding depends upon the switching duty cycle of the switch. For this purpose, the power switching converter comprises a control circuit for driving the switch; the control circuit is configured to receive a feedback signal and operate the modification of the width of the control pulses of the switch in response to the feedback signal.
The feedback control is provided by means of an optocoupler or an auxiliary winding. In the last case, the auxiliary winding gives an image of the output voltage, being directly in phase with the secondary winding.
In conditions of light load the power switching converter is typically made operate in the so-called “burst-mode”. With this operating mode the converter operates intermittently, with series (bursts) of switching cycles separated by time intervals during which the converter does not switch (idle time). When the load is such that the converter has just entered burst-mode operation, the idle time is short; as the load decreases, the duration of the bursts decreases as well and the idle time increases. In this way, the average switching frequency is considerably reduced and, consequently, the switching losses associated to the parasitic elements in the converter and the conduction losses related to the flow of reactive current in the transformer are reduced. The duration of the bursts and the idle time are determined by the feedback loop so that the output voltage of the converter always remains under control.
In the case wherein the feedback of the output voltage is formed by means of an auxiliary winding, the auxiliary winding also provides the supply voltage to the control circuit by means of a capacitor which sets a supply voltage, said capacitor being coupled with the auxiliary winding through a diode.
The minimum frequency of the burst-mode operation is determined by the control circuit of the switch; during the burst-mode operation, the control circuit periodically forces the switching-on of the switch with a certain “restart” frequency in order to receive the feedback signal. Thus, the power switching converter provides a fixed power which is independent from the load and this power needs to be dissipated to avoid that in case of low or zero load the converter goes out of regulation. To this purpose, a dummy load is typically used.
The power to dissipate mainly depends on the “restart” frequency, which cannot be chosen too low. In fact, during the time period between two subsequent commutations of the switch, the control circuit is not able to respond to an eventual variation of the load at the output terminal. Only when a commutation of the switch occurs the converter responds by providing to the load the required power. To overcome this problem, a known solution is to use a so-called wake up circuit configured to force a switching-on of the switch when the output voltage value is low during burst-mode.
The wake up circuit must interpret different load scenery and consequently to provide to the load the right power, maintaining good control performance and avoiding drawbacks like acoustic noise caused by a non-controlled voltage supply when the switching converter operates in burst mode with frequencies close to the audible range.
In particular, when the switching converter operates in burst mode the output voltage value at which the wake up circuit occurs is lower than the regulated output voltage value of a certain percentage. In this way the control circuit could provide the maximum power to the load to bring rapidly the output voltage at the regulated value. This raises two problems: acoustic noise due to the excessive current on the transformer; and a non-controlled ripple of the output voltage due to a delay of response of the wake up and control circuits.
In the case wherein the feedback signal derives from an auxiliary winding and the auxiliary winding also provides the supply voltage to the control circuit by means of a capacitor which sets a supply voltage, for very low loads and in the cases in which the average consumption of supply voltage is greater than the average consumption of output voltage, the restart frequency is very low and, due to a prolonged switching inactivity, the capacitor which defines the supply voltage of the control circuit could be excessively discharged. Sometimes is possible that the capacitor could have a value in which the supply voltage is very close to the Undervoltage-lockout (UVLO) threshold of the converter. In these conditions, the feedback voltage depends on both the output signal and the supply voltage, case in which the sampled feedback voltage is even lower than said percentage. This causes a prolonged high power phase and a higher ripple of the output voltage.